There are two basic types of metal detectors: induction balance and pulse induction as the industry has come to know these two types.
Induction balance (IB), which is a traditional metal detector, transmits (or induces) a varying current in the medium (ground) to be searched. Typically it was a sine wave, and then the induced current more recently became a combination of several frequencies, or derived frequencies from a square wave. In that type of detector, the ideal place to look for the target (coin) signal is when the transmit or inducing current is not changing since a changing current causes the ground mineral (iron or ferrite) to create a signal, often obscuring the desired target object.
In the pulse induction (PI) detector, typically there is no looking for the target until after the pulse has ended and the current has returned to zero, leaving only the decaying field current from the desired eddy current object.
The induction balance detector is very sensitive to target objects when the searching window is centered about an average point in the excitation curve where there is no change in current, although that may be at a peak current. But, small variations in the ground nature and in the excitation current can also create small, but undesired false signals.
Whereas, the traditional pulse induction detector has no excitation current during the interrogation window, so the largely ferrous response does not obscure the fading target signal, unless second order elements exist, such as remnant ground minerals that exhibit a diminishing permanent magnetism. But, the pulse detector requires a settling time after the current goes from a high value to zero, which means the interrogation for target is slightly delayed, obscuring the smaller targets with fast eddy current decay time.
Needs exist for improved metal detectors.